High frequency induction heater



y 1955 J. R. CATER HIGH FREQUENCY INDUCTION HEATER 3 Sheets-Sheet l F edJuly 8.

INVENTOR o a/m/ A. C475 BY 5%, 71 14 A 62440;

ATTORNEYS July 24, 1956 J. R. CATER 2,756,313

HIGH FREQUENCY INDUCTION HEATER Filed July 8, 1953 3 Sheets-Sheet 2 H Tq @5225 H 7'0 m4 TlE.].E.

ATTORNEYS July 24, 1956 J. R. CATER HIGH FREQUENCY INDUCTION HEATERFiled July 8, 1953 TlElU.

3 Sheets-Sheet 3 ATTORN EYS United States Patent HIGH FREQUENCYINDUCTION HEATER John R. Cater, Nutley, N. J assignor to Tung-SolElectric Inc., a corporation of Delaware Application July 8, 1953,Serial No. 366,831

3 Claims. (Cl. 219-10.75)

The present invention relates to high frequency heating and, moreparticularly, to high frequency induction heating and comprises anefiicient induction heater particularly designed for use where it isdesired to concentrate intense heat over a small area. The new heatingdevice may be constructed of relatively few parts, can be readily madeportable and is safe to operate.

In many applications it is important to be able to concentrate heat at adefinite small location as, for example, when a crystal is to be mountedon the end of a metal pin as in the manufacture of transistors orcrystal diodes, or when the getter in a sublniniature tube is to bevaporized. The new induction heater includes low resistance windingsforming part of an oscillatory circuit and so constructed and coupled asto concentrate the high frequency field at the desired work region. ineach embodiment of the invention the carrier of the high frequencyinduced currents in the neighborhood of the work region is at D. C.ground potential thus minimizing danger to the operator. In certain ofthe embodiments of the invention one carrier of the high frequencycurrents is a longitudinally split cylindrical or conical sheet memberwhich is inductively coupled to a primary winding comprising a pluralityof turns of copper tubing and serves as a single turn secondarytherefor. Associated with this secondary is a field localizer which maybe a work loop providing a low impedance path for the high frequencycurrent, in parallel with the paths provided by one surface of thesecondary sheet member and in series with the path provided by theopposite surface of the secondary sheet member. The Work loopconcentrates the high frequency field for application of heat at thedesired location. Alternativeiy the field localizer may be a currentconcentrator in the form of a radially split and centrally apertured capor cover for the cylindrical or conical sheet member. In the preferredembodiment of the invention, the single turn sheet member isconductively, as Well as inductively, coupled to the primary windingthus forming therewith an autotransformer and the field localizer is awork loop bridging the longitudinal slot of the sheet member andpreferably constructed as an integral part of a jig for holding the workto be heated. in still another embodiment of the invention the highfrequency current carrier is a flat spirally wound metallic ribbonforming the inductance element of a tank circuit of an oscillator. Thisis inductively coupled to a flat, slotted, centrally apertured discproviding a central highly concentrated high frequency field.

For a better understanding of the invention and of various embodimentsthereof, reference may be had to the accompanying drawings, of which:

Fig. l is an exploded view of an auto-transformer type of inductionheater embodying the invention and representing the presently preferredform thereof;

Fig. 2 is a side view, partly broken away, of the assembledauto-transformer of the heater of Fig. 1;

Fig. 3 is an isometric view of a combined work loop 2,756,313 PatentedJuly 24, 1956 and jig which may be used with the auto-transformer ofFigs. 1 and 2 as an alternative to the simple form of work loop shown inFig. 1;

Fig. 4 is a schematic diagram of a circuit incorporating theauto-transformer of Figs. 1 and 2, the single turn secondary not beingshown in the figure;

Fig. 5 is a side view of a conical induction heater of auto-transformertype embodying the invention, the associated circuit being indicateddiagrammatically;

Fig. 6 is an enlarged sectional detail view taken on the line 6-6 ofFig. 5;

Fig. 7 is an isometric view of an induction heater representing anotherembodiment of the invention;

Fig. 8 is an isometric view of the spirally wound coil of the heater ofFig. 7;

Fig. 9 is a view similar to Fig. 8 but showing the coils partly unwoundto illustrate the construction thereof;

Fig. 10 is a schematic diagram of a circuit incorporating the heater ofFig. 7;

Fig. 11 is a schematic diagram of an induction heater and oscillatorycircuit therefor representing still another embodiment of the invention;and

Fig. 12 is an exploded view of the transformer and current concentratorof the embodiment of the invention illustrated in Fig. 11.

The invention will first be described with reference to theauto-transformer type of induction heater illustrated in Figs. 1 through4. In this embodiment of the invention a helical coil 2 of copper tubinghas an upper turn 2' which has been spread to give it a larger radius ofcurvature than the remaining turns. A copper sheet 4 is disposed aboutcoil 2 in the form of a cylinder but with its meeting edges turnedradially outward to provide a longitudinal slot 6. The upper edge of thesheet 4, through an arc of 300 or more, is soldered to the top turn 2 ofcoil 2, the sheet thus serving as a single turn secondary conductivelyand inductively coupled to the primary winding 2. Beyond turn 2 thetubing is brought down to the base of the coil 2 internally of the sheet4 and adjacent the slot 6 as indicated at 8. Soldered to the lower endof the tubing part 8 is a coil 10 comprising a few turns of coppertubing and threaded through most of the turns of coil 2 and through coil10 are wires 12 and 14, respectively, which are sheathed in a suitableinsulating material, as for example the tetrafiuoroethylone polymermanufactured by E. I. du Pont de Nemours & Co. under the trademark Teflon. The sides 16 and 18 of the sheet 4 defining the slot 6 extendradially outward from the axis of the coil 2 for a distance sufficientto accommodate steel blocks 20 and 22, each of which is provided with aseries of horizontal slots 24 which, together with the adjoining wall 16or 18, define passages for reception of the prongs of a work loop 26.The walls 16 and 18 beyond the blocks 2% and 22 are curved smoothly toprovide wing sections 16 and 18' in each of which is formed a pluralityof holes 28 which aline with the passages provided by the slots 24. Theblocks 20 and 22 are soldered to the walls 16 and 18 and to the wingsections 16' and 18' and each is smoothly curved at one corner toconform with the curvature of the junction between wall 16 and wing 16'or wall 18 and Wing 18. The prongs of work loop 26 are held in anyselected pair of slots 24 by means of set screws (not shown) which areinserted into threaded openings 30 in the outer walls of the steelblocks and which intersect the slots 24. The walls 16 and 18 and flanges16 and 18 increase the length of the conductive path on the outersurface of the secondary, in parallel with the work loop 26. These wallsand flanges also serve to shield the steel blocks 20 and 22 from theradio frequency field and hence permit the use of into one or anotherpair of the openings 28 and alined slots 24, provides a conductive pathbetween the wings 16 and 18 for the high frequency currents induced inthe single turn secondary comprising the sheet 4, the wall sections 16and 18 and the wing sections 16' and 18'. The current path through thework loop 26 is thus one of low impedance as compared to the path inparallel therewith, namely, the outer surfaces of the sheet 4 and wallsadjoining the slot 6. A high frequency field is thus concentrated in theneck 26' of the work loop 26 and any small conductive object to beheated, held within this region, will be subjected to intense heat. Thesheet 4 is electrically grounded and therefore the work loop 26, whencoupled to the sheet 4, is at D. C. ground potential. Accordingly, thework loop may be incorporated in a jig for holding the work and such ajig is illustrated in Fig. 3.

The jig of Fig. 3 includes a pair of prongs 32 which, like the prongs ofthe work loop 26, are adapted to be inserted into any pair of holes 28in the walls 16 and 18. The inner ends of the prongs are soldered to acopper plate 34 having a central small aperture 36 and a slot 38connecting the aperture 36 with one side of the plate 34 intermediatethe locations at which the prongs 32 are soldered. The plate 34 supportsa block 40 upon the upper end of which is mounted a plate 42 carrying anarbor 44 for holding a work element, the arbor being preferably movabletoward and away from the aperture 36 in the plate 34. Plate 34 issupported on a U-shaped bracket 46, in the base of which, beneath theopening 36, is mounted a work element support 48, also preferablyadjustable toward and away from the opening 36.

Instead of providing a work loop coupled to the wings 16 and 18', a workloop or current concentrator may be coupled to the upper end of thecylindrical portion of the sheet 4. For this purpose, a disc 50, havingan annular flange 52, a central aperture 54 and a radial slot 56extending from the aperture 54 through the flange 52, may be provided.The disc 50, when positioned on the upper end of the sheet 4 with theslot 56 bridging the walls 16 and 18, provides a low impedance path forthe high frequency currents in parallel with the path including theouter surfaces of the sheet 4, of walls 16 and 18 and of wings 16 and18. Thus, the high frequency field is concentrated within the aperture54 providing an area at which intense heat may be developed in a workelement. As the disc 50, when the sheet 4 is grounded, will also be atD. C. ground potential, no danger to the operator from arcing willresult when a work element is held in the aperture 54.

The circuit of the above described heater will be clear from Fig. 4 towhich reference may now be had. In Fig. 4 the turns 2a represent thecoiled tube 2, and the half-turn a represents the coil 10, certain ofthe turns 2a and the turn 100 being threaded by the insulated leads 12and 14-, respectively. One end of lead 12 is connected through a highfrequency choke 58 to a source of high voltage as, for example, 1 /2kilovolts, a condenser 60 being connected between the high voltage sideof the choke and ground. The other end of the wire 12 is connected tothe anodes of a pair of triodes 62. One end of the wire 14 is connectedthrough an RC network 64 to ground and the other end is connected to thegrids of the tubes 62 through a pair of circuits each comprising aresistor 66 and an inductor 68 in parallel therewith. The junction ofturns 2a and 10a is grounded and an R. F. bypass condenser 70 isconnected between the high voltage end of the conductor 12 and thegrounded junction of the turns 2a and 10a. A pair of condensers 72 and74 are connected in series between the high end of plate coil 2a and theungrounded end of the grid coil 10a. The cathodes of the triodes 62 areconnected in parallel across a suitable source of potential connected toterminals 76.

Across the terminals 76 is connected a resistor 78 and a circuitcomprising series connected condensers 8d. The junction of thecondensers 88 is grounded as is the center of resistor 78. For testpurposes, a terminal 82 connected through a resistor 84 to the highpotential end of the RC network 64 may be provided.

The operation of the above described oscillatory circuit will be readilyapparent. The inductive element of the tank circuit comprises the coil2:: in the plate circuit of the triodes and the coil 10:: in the gridcircuit of the triodes. Condensers 72 and 74 comprise the capacitiveelement of the tank circuit. When a high voltage is impressed upon theanodes of the triodes 62, the circuit will oscillate due to theinductive coupling between the coils 2a and 10a in the plate and gridcircuits, respectively, and high frequency currents will flow in thesingle turn 4 and work loop connected thereto.

In Fig. 5, a high frequency induction heater of the general type ofFigs. 1 and 2, but of a slightly dilferent geometric form is disclosed.In this embodiment of the invention, a copper tube is wound into acomically shaped coil 86 and to the upper turn 86 thereof is soldered aconical sheath 88 which corresponds to the one turn secondary 4 of Fig.2. The sheath 88 is continuous about the coil 86 except through a smallangle where the side edges of the sheath are turned outwardly to providean opening 90 corresponding to the slot 6 of the embodiment of theinvention illustrated in Fig. l. A length 92 of the tubing beyond theturn 86' is brought down along one side of the opening 90 and issoldered to the sheath 88. Preferably a layer of mica, or the like, isprovided within the opening 90 to insure that the gap will not beelectrically bridged. The length 92 and sheath 88 are grounded asindicated at 93. To the lower end of the length 92 is soldered one endof a copper coil 94 corresponding to the grid coil 10a of Fig. 4. Aninsulated wire 96, one end of which is connected through a highfrequency choke 98 to a source of high voltage, and the other end ofwhich is connected to the anode of a triode 100, is threaded through thelength 92 of copper tubing and through the turns 86 thereof. A secondinsulated wire 102, one end of which is connected to ground through aresistor 104 and the other end of which is connected to the grid of thetube 100, is threaded through tubing 94. Tank condensers 106 areconnected in series across the lower ends of the coils 86 and 94. Abypass condenser 108 for the high frequency currents is connectedbetween ground and the junction of wire 96 with the choke 98. With thisembodiment of the invention, because of the conical shape of the coil 86and sheath 88, the high frequency field is more or less directional andconcentrated substantially in a region below the apex of the conedefined by the heater. Consequently, this form of device is ofparticular value when the work element cannot be brought into closejuxtaposition with the heater, as for example, when the element to beheated is within an enclosure such as a vacuum tube. As the sheath isgrounded both for direct current and for high frequency current, it maybe held in the hands of an operator and directed at the work element forconcentration of the heat at the desired locality. Obviously anadditional triode in parallel with triode could be provided as in thecircuit of Fig. 4. If the sheath 86 is brought substantially over theupper end of the winding 86 to concentrate the induced currents in thatregion, or if a current concentrator of the type indicated at 50 in Fig.l is added, the heating region can be squeezed into a very small areaand the apparatus used for heating objects located substantially at theupper end of the coil.

The embodiment of the invention illustrated in Figs. 7 through 10differs from those heretofore described pri marily in that the lowresistance carrier of the high frequency currents is in the form of aflat ribbon rather than in the form of a tubular element. The physicalconstruction of this embodiment of the invention will be understood fromFigs. 7, 8 and 9 and the circuit will be understood by reference to Fig.10. A flat copper ribbon 110, the end terminals of which are indicatedat 112 and 114, is wound in a continuous spiral and lashed to the edgesof the ribbon for certain of the turns thereof are insulated wires 116,118, and 122. One end of the insulated Wire 116 is brought outsubstantially midway of the spiral for connection through a choke 124 toa source of high potential, and one end of the insulated wire 118 isbrought out at substantially the same point for connection to wire 116and to the choke 124. Wires 116 and 118 which are lashed along the sameedge of the ribbon extend in opposite directions and are brought outadjacent the ends of the copper ribbon 110 to terminals 126 and 128,respectively. Terminal 126 of Wire 116 is connected to the anode of atriode and terminal 128 of wire 118 is connected to the anode of atriode 132. The insulated wires 120 and 122 are lashed to the ribbon 110along the edge of the ribbon opposite to that to which the anode wires116 and 118 are lashed and through a lesser number of turns of thespiral. The terminals of Wire 1120 are indicated at 134 and 136 and theterminals of wire 122 are indicated at 138 and 140. Terminal 134 isconnected to the grid of triode 132 and terminal 140 is connected to thegrid of triode 130. Terminals 136 and 138 are connected together and toground through a resistor M2. The copper ribbon 110 adjacent itsmidsection is grounded as indicated in Fig. 10, the ground lead notbeing visible in the view of Fig. 8. The heater is completed by a highconductivity metal disc 144 which is closely spaced to the spiral coilby means of an effective insulating sheet 145 such as mica and mayconveniently carry a jig for holding a work element. The disc 144 has acentral aperture 146 and a radial slit 147 extending therefrom to theperiphery of the disc. The disc is grounded at a point on its peripherydiametrically opposite to the end of the slit. In the particularembodiment of the invention illustrated in Fig. 7, a hub 148 ofdielectric material is centrally mounted on the underside of the disc tofit within the space defined by the inner turn of the coil and disposedin the aperture 146 of the disc 144 and supported in the hub 148 is acylindrical insert 149, also of dielectric material but of smallerdiameter than the hub 146. The disc 144 serves to concentrate the highfrequency field at the center of the disc so that a conductive elementplaced upon the insert 149 will be subjected to intense heat.

In the particular embodiment of the invention illustrated in Fig. 7, anarm 158, having a slotted opening at one end for holding a work elementis pivotally mounted between furcations of a bifurcated bracket 152rotatably mounted on the disc 144. Adjacent the inset 149 on the disc Ml is mounted a brace or step 154 upon which the work-holding end of thelever may be braced during the heating operation and also mounted on thedisc 144 is an adjustable step 155 for the lever to assure exactpositioning of the work element. Obviously any other suitable means forpositioning an element to be heated over the inset 149 and forsupporting the insert 1--9 could be provided. The oscillatory circuit ofthe pancake type of induction heater above described is of the push-pulltype as is obvious from Fig. l(). The tank circuit comprises the coil ofribbon 110 and condensers 156 and 158 connected in series across the endof the ribbon. The anode circuits of the tubes 130 and 132 areinductively coupled to the winding 110 through different sectionsthereof and the grid circuits are inductably coupled with the anodecircuits of the opposite tubes.

Still another embodiment of the invention, similar in some respects tothat of Figs. 1 and 2, is illustrated in Figs. 11 and 12. The inductionheater of Figs. 11 and 12 is of the transformer type, but not anauto-transformer as in the case of Figs. 1, 2 and 5. A copper coil 160is connected at one end to the anode of a triode 162 and at its otherend to the anode of a triode 164. An insulated Wire 166 is threadedthrough the tubing 160 for a few turns adjacent its midsection andcross-connected at its ends to the grids of triodes 164 and 162. A pairof condensers 168 are connected in series across the ends of the coil160. The midpoint of the copper coil 162 is connected through a choke170 to a source of high voltage and the midpoint of the wire 166 isconnected through a resistor 172 to ground. A bypass condenser 174 isconnected between the high frequency source and ground. Positionedwithin the copper coil 160 is a generally cylindrical element 176 whichhas the outer configuration of a longitudinally slotted cylinder, theslot being indicated at 178 in Fig. 12.

The internal structure of the member 176 comprises longitudinal ribs orbaffles 180 and 182 which extend radially inward from the inner wall andterminate short of the axis and short of inwardly extending wallportions 184 and 186 defining the slot 178. The wall portions 184- and186, which extend nearly to the opposite side of the member are curvedoutwardly and then inwardly around the axis as is indicated at 188. Theabove described internal structure insures a relatively long internalpath for the induced high frequency currents. A current concentrator19%) generally similar to concentrator 50 of Fig. l and comprising acentrally apertured and radially slotted cap completes the assembly andprovides a low impedance current path in parallel with the longer pathsprovided by the member 176. The central aperture 192 is the work regionat which an element to be heated is placed during the heating operation.The structure of Figs. ll and 12 and associated push-pull circuit may beconveniently mounted in a portable unit with the member 176 grounded forminimizing electrical shock to the operator.

The invention has now been described with reference to variousembodiments, each of which has been found in use to be efficient andeffective for localized heating of small parts.

The following is claimed:

1. A high frequency induction heater comprising a helically wound tubeof high conductivity, a cylindrical longitudinally split sheet memberencompassing said tube and conductively connected to an end turn thereofto serve as a secondary winding inductively and conductively coupledthereto, a looped conductor of low resistance connected to said sheetmember at opposite sides of the slot therein to provide a current pathof low impedance in parallel with the path comprising the outer surfacesof said sheet member and to define a work area, an oscillatory circuitincluding said helically wound tube as one element thereof, the edges ofsaid sheet adjacent the longitudinal slit extending radially outwardsand then diverging smoothly to provide wing portions and meansassociated with said wing portions for detachably connecting said loopedconductor thereacross.

2. The induction heater according to claim 1 wherein said last mentionedmeans include steel members welded to said outwardly extending edges ofsaid sheet member and to said wing portions and protected thereby fromradio frequency fields.

3. A high frequency induction heater comprising a helically wound tubeof high conductivity, a cylindrical longitudinally split sheet memberencompassing said tube and conductively connected to an end turn thereofto serve as a secondary winding inductively and conductively coupledthereto, a looped conductor of low resistance connected to said sheetmember at opposite sides of the slot therein to provide a current pathof low impedance in parallel with the path comprising the outer surfacesof said sheet member and to define a work area, an oscillatory circuitincluding said helically wound tube as one element thereof, saidoscillatory circuit in cluding at least one electronic tube having ananode, a grid and a grounded cathode, an insulated wire threaded throughturns of said helically wound tube and connected at one end to saidanode and at its other end through a high frequency choke to a source ofhigh potential, a second helically wound tube connected at one end tothe same end of said first helically wound tube as that to whichsaid-sheet member is connected and capacitatively coupled at its otherend to the other end of said first helically wound tube and an insulatedwire threaded through said second helically wound tube and connected atone end to said grid and at its other end through an impedance toground.

References Cited in the file of this patent UNITED STATES PATENTS BrownMar. 7,

Grumel Aug. 1,

Litteli Mar. 10,

Wilson July 20,

FOREIGN PATENTS Great Britain Apr. 4,

Great Britain Oct. 1,

